1. Field of the Invention
The present invention relates to a system for controlling exhaust gas sensor having a heater, the exhaust gas sensor being installed in an exhaust pipe of an internal combustion engine.
2. Description of Related Art
In an electronically controlled internal combustion engine, an exhaust gas sensor that detects oxygen density or an air-fuel ratio in exhaust gas is disposed in an exhaust pipe. An amount of fuel to be injected into the engine is controlled based on the oxygen density or the air-fuel ratio in the exhaust gas, so that the air-fuel ratio in the exhaust gas becomes equal to a target level. Generally, detection accuracy of the exhaust gas sensor is not sufficiently high before a temperature of the sensor reaches activation temperature. Therefore, the exhaust gas sensor is heated by a heater installed in the sensor.
Vaporized water that is formulated in combustion reaction of fuel and air is contained in the exhaust gas. The vaporized water in the exhaust gas condenses on the exhaust gas sensor when temperature in the exhaust pipe is low. If the exhaust gas sensor is heated to activate it by the heater immediately after the engine is started, the heated sensor element is partially cooled down by the condensed water. This may cause cracking of the sensor element.
To cope with this problem, JP-A-2003-269231 proposes to prohibit heating of the sensor until it is determined that water drops condensed on the sensor are scattered away by an exhaust gas flow in the exhaust pipe. In this system, however, activation of the sensor is delayed, and a feedback of the detected air-fuel ratio in the exhaust gas is delayed. This results in worsening in the emission quality.
JP-A-2003-328821 proposes a system, in which the exhaust gas sensor is heated immediately after the engine is started if it is determined that temperature in the exhaust pipe is high enough to prevent water condensation. On the other hand, a period of time, in which the heater operation is prohibited (referred to as a heater-off period), is set according to cooling water temperature if it is determined that temperature in the exhaust pipe is low. In this system, however, the heater-off period is not adequately set because it is set based on only the cooling water temperature. There is such a tendency that the heater-off period is set unnecessarily long, resulting in worsening the emission quality.
Further, JP-A-6-90167 proposes a system, in which a temperature sensor is installed in the exhaust gas sensor, and heating of the exhaust gas sensor is prohibited when the temperature of the exhaust gas sensor detected by the temperature sensor is lower than a predetermined level. In this system, however, it is necessary to install the temperature sensor in the exhaust gas sensor, making the exhaust gas sensor expensive.
Further, JP-A-2002-48749 proposes another system. In this system, an amount of heat or a temperature in the exhaust gas is calculated according to operating conditions of the engine, and a temperature of the exhaust pipe is calculated based on a mathematical heat conduction model between the exhaust pipe and the atmosphere. After the calculated temperature reaches a temperature at which no water condensation occurs, the exhaust gas sensor is heated by the heater. In this system, however, a computer load for calculating the exhaust pipe temperature becomes high, resulting in complication of a control system.